1. Technical Field
The present invention relates to a test cell for testing fluids at elevated pressures. More specifically, the test cell of the present invention comprises a pressure-neutral cylinder for use in pressure, volume and temperature (pVT) studies of reservoir fluids and their properties in the laboratory and in the field.
In pVT cells and condensate cells, petroleum fluids can be studied at varying pressure and temperature, simulating the conditions in oil reservoirs before and during production. Typically, these fluids contain gas. The change in fluid density (compressibility) and the tendency for the gas to come out of solution at decreasing pressure are of particular interest.
pVT cells are optimized to study oils with dissolved gas, while condensate cells are optimized to study light oils with a high gas to oil ratio. In the following, they are both denoted pVT cells.
2. Description of Related Art
Until the late eighties, the method for controlling the pressure in these pVT cells was to pump mercury in and out of the cell, mercury being considered as inert with respect to the petroleum fluids.
There were however some health risks involved in the handling of mercury at high pressure and temperature, and this method has to a large extent been replaced by other methods for changing the volume in pVT cells. Several of the new designs are based on cylindrical cells with a sealed piston that can be moved by either direct mechanical drive or hydraulic drive.
A problem with the piston solution is that the diameter of the cell will change with pressure, and thus the clearance between piston and wall will change, which makes rather high demands on the seals.
In order to solve the problem of varying clearance with pressure, a relatively thin inner cylinder, which is in contact with a dynamic seal on the piston and with the fluids, is enclosed in a thick-walled high pressure cylinder. The space between the cylinders is filled with hydraulic fluid, and is connected to the same line that provides such fluid under high pressure to hydraulically control the piston position. Thus it is ensured that the (differential) pressure across the inner cylinder is negligible.
This solution gives the following advantages:
1. The material of the inner cylinder can be selected independently to meet various specifications. The material of the inner cylinder (e.g. Hastelloy C, glass, Inconel) can be selected to be chemically compatible with the fluids while the outer cylinder needs only to be strong enough to meet the pressure specification (e.g. high strength steel) or a combination of weight and strength specifications (e.g. Ti-6A1-4V).
2. The diameter of the inner tube does not change with pressure, and the volume of the test fluid chamber is therefore only dependent on the position of the piston which can be monitored directly. This solution is applied in the so-called “DBR Jefri” cells with utilization of a pressure-neutral inner cylinder made of a glass material, and with external connection for the pressure outside the inner cylinder and the pressure behind the piston.
If the piston position accidentally is at the bottom of the cylinder, a pressure difference across the inner cylinder wall may occur due to elevated pressure in the inside test fluid (caused by temperature increase or charging with more test fluid), or by a falling pressure in the hydraulic system. This pressure difference might burst or cause plastic flow of the inner cylinder wall, depending on the cylinder material being brittle or ductile.
While the DBR solution provides a pressure-neutral inner cylinder and chemical compatibility with test fluids, both material deformation properties and clearances are such as to allow the inner cylinder to deform and/or break.
Hence, an alternative apparatus to those described above is needed to perform pVT studies without the risk and inconvenience of bursting or deforming the inner cylinder.